RRDtool

From the Website:

"

Logging & Graphing

The industry standard data logging and graphing application. Use it to write your custom monitoring shell scripts or create whole applications using its Perl, Python or PHP bindings. Create graphs like this:

RRD is the acronym for Round Robin Database. It is a system to store and display time-series data (i.e. network bandwidth, machine-room temperature, server load average)."